In living organisms, biological processes generate different types of output signals, which are generally referred to as biosignals. Biosignals may be electrical, mechanical or chemical.
Bioelectricity is a broad field including measurements of biopotentials and bioimpedance. Biopotentials cover electricity created in life processes internal in tissues, while in bioimpedance measurements electric currents are supplied from an external source outside living tissues. Biopotentials thus generally refer to active processes, such as excitation of nerve and muscle tissues, whereas bioimpedance is related to passive properties of the tissue, such as the properties of the skin. However, these passive properties can also be related to electrical or other processes internal in tissues, even though the measurement does not directly utilize the electricity generated internally in tissues.
Neuromonitoring is a subfield of clinical patient monitoring focused on measuring various aspects of brain function and on changes therein caused by drugs commonly used to induce and maintain anesthesia in an operation room or sedation in patients under critical or intensive care.
Electroencephalography (EEG) is a well-established method for assessing brain activity by recording and analyzing the weak biopotential signals generated in the cortex of the brain with electrodes attached on the skin of the skull surface. The EEG has been in wide use for decades in basic research of the neural systems of the brain as well as in the clinical diagnosis of various neurophysiological diseases and disorders.
Electrocardiography (ECG) is another well-established method for assessing cardiac function by recording and analyzing the rather strong biopotential signals generated in the heart. The electrodes are attached on the skin of the chest with more peripheral references. The ECG is commonly used for diagnosing cardiac dysfunctions, various cardiac and circulatory diseases, and arrhythmia.
Electromyography (EMG) is a method for recording electrical biopotentials of muscles. In an EMG measurement, the electrodes are attached on the surface of the skin at a muscle group. An EMG signal is often recorded from the skull of the patient, whereby the recorded signal indicates both the activity of the facial muscle (fEMG) and the brain (EEG). As the frequencies of the EMG spectrum are usually high and above the frequencies of the brain activity, the signal components can be separated by methods of signal processing or spectral analysis from the EEG signal.
One of the special applications to which a significant amount of attention has been devoted during the past few years is the use of processed EEG signals for the objective quantification of the brain function for the purpose of determining the level of consciousness. The basic idea is to automatically detect if the subject or patient is asleep or awake. Specifically, this has become an issue, both scientifically and commercially, in the context of measuring the depth of anesthesia during surgery. The concept of the adequacy of anesthesia, which is a broader concept, further includes various other aspects relating to the quality of anesthesia, such as the state of the autonomic nervous system (ANS), and more specifically analgesia, i.e. loss of sensation of pain.
The need for reliably monitoring of the adequacy of anesthesia is based on the quality of patient care and on economy related aspects. Balanced anesthesia reduces surgical stress and there is firm evidence that adequate analgesia decreases postoperative morbidity. Awareness during surgery with insufficient analgesia may lead to a post-traumatic stress disorder. Prolonged surgical stress sensitizes the central pain pathways, which post-operatively increases patient pain and secretion of stress hormones. Low quality pre- and intra-operative analgesia makes it difficult to select the optimal pain management strategy later on. More specifically, it may cause exposure to unwanted side effects during the recovery from the surgery. Too light an anesthesia with insufficient hypnosis causes traumatic experiences both for the patient and for the anesthesia personnel. From economical point of view, too deep an anesthesia may cause increased perioperative costs through extra use of drugs and time, and also extended time required for post-operative care. Too deep a sedation may also cause complications and prolong the usage time of expensive facilities, such as the intensive care theater.
The assessment, measurement, or control of the different components of anaesthesia is ‘a line drawn in water’, as the drugs used in anaesthesia are often unspecific and influence many components simultaneously. The cortical components, i.e. hypnosis, amnesia and perception of pain and conscious control of movements, mainly refer to the activity of the cortex and integrity of the cortical evaluations of sensory afferent inputs and the ability to store information and control the body. Loss of consciousness, i.e. loosing responses to non-noxious sensory stimulations, such as spoken commands, is dominantly related to the overall suppression of cortical processing and awareness.
During the past few years, several commercial devices for measuring the level of consciousness and/or awareness in a clinical set-up during anesthesia have become available. These devices, which are based on a processed one-channel EEG signal, have been introduced by Aspect Medical (Bispectral Index), by Datex-Ohmeda (Entropy Index) and by Danmeter (an auditory evoked EEG potential monitoring device, AAI™). At present, the situation with the assessment of the cortical activity and integrity is considered satisfactory, though not resolved for all applications.
As to the central nervous system (CNS), the assessment or measurement of the suppression of the sub-cortical activity and integrity is far more unsatisfactory. No commercial devices exist for this purpose. This is mainly because the sub-cortical components are not represented in any single bioelectrical or other signal, in contrast to that the EEG almost alone may represent the cortical activity. The suppression of the sub-cortical components is demonstrated in at least two ways: first, in suppression of the sensory and pain pathways at sub-cortical level (i.e. suppressions of the afferent neuronal signaling) and, second, in suppression of the autonomic nervous system (ANS) control and reflexes (i.e. suppression of the efferent neurons and evaluations needed for efferent control at sub-cortical level).
The sub-cortical integrity of the afferent input, ANS evaluations, and efferent output is best tested with noxious stimulations and responses, as these are mainly processed and modulated in the brainstem and spinal levels. Further, as analgesic or antinociceptive drugs also have their main effects at these sub-cortical levels, the relationship between the analgesics, mainly opiods, and the suppression of the pain pathways and consequent noxious event responses exists.
International patent application WO 02/32305 discloses a method and device for ascertaining the cerebral state of a patient. In this disclosure, a measure derived from EMG signal data enhances and confirms the determination of the hypnotic state made using EEG signal data. As the EMG data may be computed more frequently than the EEG data, this renders ascertaining changes in the hypnotic state of the patient more rapid. In this method, the (facial) EMG thus alone reflects the suppression of the nociceptive pathways. State entropy (SE), which is calculated in the low frequency band up to 32 Hz, is dominated by the cortical EEG activity, while response entropy (RE), which also includes the high frequencies, represents both the cortical and muscle activity. The difference RE-SE is, therefore, a measure of the (f)EMG power, which will increase at nociception and is therefore a good measure of the suppression of the pain pathways. However, the above-mentioned dependency on the medication of the patient may render the method unusable in certain situations. As the (facial) electromyography signal is affected by neuro-muscular blocking agents (NMBAs), which suppress signaling at the nerve-muscle junctions, the EMG component of the measurement may vanish and render the method unusable, if the medication of the patient includes neuro-muscular blocking agents. It shall also be emphasized that the difference RE-SE is not specific to the suppression of the pain pathways but also reflects the overall activity following any arousals in the CNS. The fEMG signal is thus activated after non-noxious stimulation such as auditory stimulation and it is difficult to assess if any noxious components are present in the facial muscle response.
The present invention seeks to alleviate or eliminate the above drawbacks and to bring about a complementary method which may be used in situations where methods resting on EMG signal data are not usable.